1. Field of the Invention
The present invention relates to an interference device and method for observing phase information and, more particularly, to a method for observing phase information using waves with the amplitude difficult to split such as electron beams and an interference device for observing phase information.
2. Description of the Related Art
So far, electron-beam holography has been known and used for observing the image of a very minuscule phase object. The principle will now be explained briefly with reference to FIG. 3. As illustrated, a plane electron wave 1 having high coherence enters along the optical axis, on one side of which a specimen 2 is located. The electron wave 1 then travels through the specimen 2, so that the wavefront can be split into an object wave modulated by the specimen 2 and a reference wave that does not transmit through the specimen 2. These waves are focused once through an electron objective 3 to form an image on an intermediate image-formation surface 5. If there are electron biprisms 4 between the objective 3 and the intermediate image-formation surface 5, then the object wave passing on one side of a centrally located filament and the reference wave on the other side are bent across the optical axis and superposed on each other on the image-formation surface 5 to form interference fringes, which are then magnified through an electron lens 6 to record them on a photographic film 7, thereby producing a hologram 7. On the thus produced hologram 7, the phase of the object wave spatially magnified by the electron lens is recorded in the form of an interference fringe displacement. The recorded phase of the object wave may be measured by interference between the wavefronts of the optically reconstructed hologram 7 and plane waves, for instance. Or, alternatively, the electron-beam hologram may be digitized for reading, so that the phase of the recorded object wave can be measured by transforming the read data by calculation.
As illustrated in FIG. 4, another well-known type of holography may be achieved by use of three crystals 11, 12, and 13. An incident electron beam 1 is diffracted by the crystal 11 to split it into a positive first-order diffracted wave and a negative first-order diffracted wave by amplitude splitting. These positive and negative first-order diffracted waves are then subjected to the negative and positive first-order diffraction through the crystal 12, respectively, so that one of the diffracted waves falls upon the crystal 13 through a specimen 2 and the other strikes the crystal 13 through vacuum. Subsequently, they are again subjected to the - and + first-order diffraction, travel through an electron lens 14, and interfere on an observation surface 15 in a superposed manner, thereby forming a hologram.
The electron-beam holography of FIG. 3 utilizing wavefront splitting has wide practical application, but has drawbacks. For instance, because the interference fringe pattern itself provides no direct representation of the phase distribution of a specimen, it is necessary to reconstruct the equiphase distribution of the specimen from the interference fringe pattern either by optical techniques or by calculation. For achieving wavefront splitting, it is also required to use an electron beam source having high coherence. Moreover, there is the need of providing a vacuum region through which reference waves can travel immediately adjacent to the location of the specimen to be observed. However, large specimens have fuzzy profiles; in other words, they are not well suited for ideal measurement.
The amplitude splitting technique of FIG. 4 has very little practical application, partly because it is difficult to obtain any pure crystal that behaves as theoretically expected, and partly because it is very difficult to achieve crystallographic alignment.